The present invention concerns mounting rims for tires, as well as the complete tire-rim assembly, and in particular, it concerns rims provided with devices to prevent unseating of the bead.
It is already known that when a vehicle is cornering, there is a force acting upon the tire sidewalls, in a direction substantially parallel to the wheel axis, the intensity of which varies depending upon the radius of curvature of the cornering and the speed of the vehicle.
This cornering force acts so as to shift the outboard tire bead from its seat on the mounting rim towards the axially inner zone of the rim, and it is resisted, to a large extent, by the inflation pressure of the tire. When the tire is at least partially deflated, for each value of this force there exists a well-determined pressure value (lower than that of normal inflation pressure), for which value the tire bead heel shifts effectively from its normal seat on the mounting rim axially inwardly to then drop into the central well of the rim, which is usually located along the meridian plane of the rim. When this takes place, the consequence is usually a loss of control of the vehicle by the driver.
For this reason, the problem of impeding the shifting of the bead heal was always important, but has become even more important after improvements made in the state of the roads, and also due to the improved performance of vehicles, that has allowed for even greater traveling speeds.
For this purpose, there are many prior art devices intended to block the bead heel upon the rim to prevent axial shifting. Among these devices are those that are more generally adopted to consist of a particular shaping, better known as "hump", formed on the rim in the proximity of the axially inner extremity of the bead seat, and extending radially outwardly for a constant height with respect to the seat of the bead heel, which is intended to constitute an insurmountable obstacle to the axial shifting movement of the tire bead.
However, this hump must be straddled by the bead heel of the tire during the step of mounting the tire onto the rim, and this mounting is done with a force that is less than that applied to the wheel when driving under cornering conditions.
It results from this, that the radial height of the hump, with respect to the bead seat surface, cannot exceed determined limits. In fact, if the height of this hump is raised so as positively to block the bead heel on the tire rim (under the above-mentioned critical conditions), the result is that the tire could not be mounted onto the rim.
The height of this hump is such as to allow it to be passed over by the tire bead heel during its mounting upon the rim by taking advantage of the deformability of the metallic bead core (which is circumferentially inextensible, but rather flexible, so much so that it can assume an elliptical configuration) and the compressibility of the elastomeric material that covers the radially inner surface of the metallic bead core.
From this there results a compromise, characterized by a certain difficulty in mounting the tire, but with a corresponding sealing capacity of the rim with respect to the tire bead heel in the instance of a partially deflated tire during cornering, up to about 60% of the value of the normal inflation.
In attempts to improve this situation, there have been proposed rims having an asymmertical hump. In other words, the height of this hump is at a maximum at a point corresponding to a determined zone of the tire bead passing gradually from the minimum value to the maximum value and vice versa, along a certain portion of its perimeters.
In actual practice, even the above-mentioned variant has not completely solved the problem. In fact, if the increase in the height of the hump along one portion of the periphery of the bead seat is carried out at the expense of having a corresponding reduction of this height along the diametrical opposite portion, so as to keep constant the circumference of the hump, with regard to the ability to mount the tire onto the rim, no advantage is had; whereas, as far as concerns resistance to bead unseating, the behavior of the rim is decidedly worsened in the portion of diminished height.
If this greater height is obtained by increasing the height of the previous symmetrical hump along a portion of its development and, consequently, by also increasing its maximum circumference, the gain, with regard to resistance to the bead's unseating, is quite costly because of the considerable increase in difficulty in mounting of the tire.
The applicant has now discovered a new way of forming these humps, on the basis of which it is possible to make a mounting rim for tires that presents simultaneously a greater ease in mounting the tire and a greater sealing ability for the tire bead in its normal seat during cornering under partially deflated tire conditions.
It is thus a first object of the present invention to form a supporting rim for tires comprising a pair of bead seats on the rim, each one connected at its axially outer extremity to a radially outwardly extending flange, at least one of said bead seats being also connected, at its axially inner extremity, to a radially outwardly extending protuberance means, said rim being characterized by the fact that said protuberance means comprises at least two circumferentially raised protuberances that are substantially continuous and axially side-by-side, each protuberance lying on a plane perpendicular to the rim axis and presenting a cross-section that is substantially circular, the center of said section being eccentric with respect to the axis of said rim, the centers of said distinct sections being destributed around the circumference of said rim, the radius of said circular section being not greater than 2% larger than the radius of said bead seat, measured along a line between that proturberance and the adjacent seat, on said axially inner extremity.
According to certain preferred embodiments, the above-mentioned protuberance means can comprise two or even three or four circumferential protuberances, with the centers of the corresponding cross-sections symmetrically disposed around the circumference of the rim. Moreover, between two successive protuberances, there is preferrably a substantially cylindrical coupling surface.
Beside this, quite conveniently, the outer profiles of said protuberances, as seen in cross-section along a plane passing through and parallel to the rim axis, are variable along the circumference of the rim, in particular, to permit the radial height of the protuberance (especially the one that is axially outermost) to be the same as that of the surface of the bead seat at a point on its circumference, and without this giving rise to any reduction in the thickness of the body of the rim in the zones that are immediately adjacent to said point.
A second object of the present invention is a complete tire-rim assembly formed with a rim according to the above-stated first object, wherein the circumference of those protuberances is not greater than the inner circumference of the bead cores in the tire bead. Moreover, in said assembly, the maximum height of said circumferential protuberances, with respect to the bead seat, extends radially outwardly no further than the radially innermost point of the bead core on the bead of the corresponding tire when mounted on the rim and inflated to normal inflation pressure for that tire.